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Stars hiding in dark matter halos made early galaxies look big

Insignificant in visible light, halo stars are big in the infrared background.

When the known galaxies and stars in the right panel are subtracted, what remains is the cosmic infrared background shown at left. Astronomers have determined much of this haze is from stars in the dark matter halos of early galaxies.

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The search for the earliest galaxies in the Universe is ongoing. Since these galaxies are far removed from us in time, they are very faint and very red-shifted, making it hard to determine how many there were and where they were distributed. To find these galaxies, some astronomers are looking at the Universe's infrared background across large patches of the sky. Fluctuations in the temperature of this infrared background are likely indicators of the first galaxies, which heated and ionized much of the gas in the Universe.

A new observation using data from the Spitzer infrared space telescope has found the expected signature of distant, faint galaxies. However, the magnitude of the fluctuations was surprisingly high: these early galaxies appeared bigger and brighter than expected from theory and observations at other wavelengths. In a new Nature paper, Asantha Cooray and colleagues suggest that much of this infrared radiation came from stars in the galactic halos, which were thought to be mostly dark matter.

Typical galaxies such as the Milky Way have two basic parts: the luminous portion (which is what we usually think of as the galaxy), and a dark matter halo that envelops it and contains most of the mass. Even though most of a galaxy's stars are in the luminous portion, the halo does contain a substantial number of stars, although they're at a much lower density. Recent studies have shown that halo stars contribute more to the total light profile of a galaxy than we previously thought.

The mass of the dark matter halos is thought to have been instrumental in drawing atoms into the first galaxies, a process that left its mark on the early Universe. The first stable atoms formed around 400,000 years after the Big Bang. As electrons joined with protons, they emitted light we now see as the cosmic microwave background. When the first stars and galaxies formed, however, their intense radiation stripped electrons from atoms again, an event known as reionization. According to theory, that is: while the ionized gas has been seen, the stars that drove it are distant and hard to observe.

However, the earliest stars and galaxies should contribute to the total infrared glow of the Universe, known as the cosmic near-infrared background (CNIB). ("Near-infrared" refers to wavelengths closest to visible light in the electromagnetic spectrum; in this case, the study was in the 1 to 5 micron range.) Much of the haze in the CNIB is from the Milky Way and known galaxies, but a significant portion is not associated with any obvious sources. Astronomers have postulated it must originate in either to dwarf galaxies (which are too small to be seen at significant distances) or faint galaxies from the early Universe.

Until the current study, no survey of the CNIB had sufficient resolution to distinguish between small (but relatively close) galaxies and the first to form in the Universe. The researchers decisively determined the overabundance of infrared emission was originating from an area that is too large to be dwarf galaxies. The surprise was that it was too large to be normal galaxies either—or at least the star-rich portions of those galaxies.

The current data strongly supports that assertion: if the halo stars were added to the galactic contribution, then they made up the difference between the expected infrared haze and the observed amount. The measured distribution of bright patches in the CNIB was consistent with galaxies as massive as the Andromeda Galaxy, but were generating light more than 7 billion years ago—over half the current age of the Universe.

These results provide significant additions to our understanding of early galaxies: their distribution, numbers, and the role of halo stars in their overall light profile.

Promoted Comments

Stupid question: when we're looking far away we see the universe when it was younger. But it was also smaller, and yet it's seen all around our present larger universe. Does this mean the early universe looks magnified? Do these galaxies appear larger, or more spaced out?

The "early Universe" is visible in every direction because the Universe has no center. So, the farther you look in every direction, the earlier in the Universe's history you see. Imagine the Universe we can see at any given distance as a balloon that has been inflating for billions of years, with us at the center. Sure, when the events we observe on the balloon actually happened, the balloon was smaller. But, the angular separations from our point of view at the center are no different than they were billions of years ago, so everything looks as far away from everything else as now.

Here's another way of looking at it. Take a very nearby spherical shell of objects and draw lines to the galaxies you see, from your position at the center of the shell. Now events in the early Universe are on a larger spherical shell, farther away in every direction. Continue drawing your lines out to this further shell. Would the galaxies appear more spaced out to you on this more distant shell? No, the angular separation is the same. The lines didn't change direction.